Tuesday, April 12, 2011

The Hidden and Undocumented 'EIGRP Too-optimal Routing' Enhancement

We often heard about routing loop and suboptimal routing, I have discovered a new hidden and undocumented EIGRP feature or enhancement yesterday - the EIGRP Too-optimal Routing enhancement.

This feature was first introduced in Cisco IOS 12.4(20)T along with another hidden and undocumented feature - the Enhanced Tunnel Interface Delay enhancement, in which the default delay for tunnel interfaces was tuned from 500000 usec to 50000 usec.

Below shows the output of a Cisco 7200 running the 12.4(20)T releases regarding the the enhancement mentioned above.

As expected, the new feature is not being implemented throughout the 12.4 releases.
And... note that the new feature is not implemented on the maintenance release for the 12.4(15)T releases as well.

Let's come back to the main topic.
With the indirect enhancement upon EIGRP due to the introduction of the Enhanced Tunnel Interface Delay enhancement, we will be gaining more hands-on experience in troubleshooting routing loop problems in DMVPN and IPVPN environments with EIGRP as the routing protocol, after replaced a router, upgraded a router, or upgraded the IOS of a router. @_@
In such environments that rely upon Tunnel interfaces heavily, the enhancement upon the delay of Tunnel interfaces greatly influence the metric calculation of EIGRP routes.
EIGRP rely upon the bandwidth and delay by default when calculating the metric.Routing loops can occur as Cisco routers running different IOS versions with and without the enhancements discussed above treat the network topology differently.

Good Point! This is the definition that I have for "K5 – Maximum Transmission Unit" for the time being - The smallest MTU along a path. MTU is included in the EIGRP Update packets but was never included in the formula used for metric calculation.